ACt 2010 not upgrading

I have Act 2010 with 2 licenses. The PC's are networked on a peer to peer network. They both share the same database. The PC on which the database resides has upgrded to version 12.1.181.0 Hot Fix 5. However, the second PC is at version 12.0.409.0 Hot Fix 4. WHen I try to upgrade to 12.1.181.0 Hot Fix 5 using the Act Update button from the Help drop down, it downloads the file, unpacks it and then says that it's not compatible with the version of Act on the PC. How do I get it to upgrade to version 12.1.181.0 Hot Fix 5?